Hi! On the search team meeting you asked me why I've decided to declense titles that we look for before searching instead of relying on a stemmer. I tried a few things and I seem to remember why now :D
The biggest problem is we're looking for exact matches of an inflected title, so we can't use the standard "no operator" search mode that uses stemming. A partial result or a result with two matching words separated by other words is useless in our case, so we have to use double quotes. I don't think it's possible to search the stemmed text with exact matching… or is it? To be honest I'm not much of an expert in Elastic, so I may be wrong here :)
Another problem is that we sometimes don't want certain words to be declensed. For example "Sejm Rzeczpospolitej Polskiej" (our parliament thing) has the "Rzeczpospolitej Polskiej" fixed in that case. We would declense it Sejm, Sejmu, Sejmowi, etc. without changing the rest of the name. This is a rare case though, as invalid forms that may stem from that are rather unlikely to occur in articles.
So that's why :)